From Dark and Darker Wiki
I can't confidently say one way or another based on intuition and game feel alone. This exact question remains to be tested and I've been too busy.
If someone else is interested and willing, it would be sufficient to continuously repeat a combo 10+ times and time it, then repeat without continuously holding the attack button. If there's a time discrepancy, then that means the combo activation period is dynamic and can end early.
Rondel Dagger is the perfect weapon for this test since attacks 2 and 3 allow you to chain a combo indefinitely.